The present invention relates to the field of the production of electricity on board vehicles or containers for example, by means of an alternator, a rectifier and a voltage-booster chopper.
A large number of on-board devices for producing electricity have been proposed, the objective being in particular to reduce the weight of the equipment.
Numerous devices have been proposed in which the voltage-booster chopper uses the self-inductance of the alternator, thereby making it possible to avoid the use of a chopper-specific coil.
The application GB-A-2289581 in particular discloses a device comprising a conventional three-phase alternator, a rectifier and a half-bridge chopper downstream of the rectifier.
Such a device is relatively complex and limited in terms of power.
The U.S. Pat. No. 5,793,625 also discloses a device of 100 W power, in which the alternator is conventional and the voltage-booster chopper comprises several FET transistors operating at a very high switching frequency.
Furthermore, the U.S. Pat. No. 4,825,139 discloses a device in which the alternator is conventional, the voltage-booster chopper using several MOS transistors.
These known devices are not entirely satisfactory, being of relatively complex structure and unsuitable for delivering power of the order of a kW at least.
To the knowledge of the applicant, only the automobile sector uses four-pole alternators, having the advantage of being simple to manufacture and of delivering a current at a relatively low frequency, and hence with small losses in the magnetic circuit.
This is because, generally, the losses in the magnetic circuit increase with the frequency of the current wave, and hence with the number of poles.
Moreover, at constant inductance value, the voltage drop dues to the phenomenon of overlap in the diodes of the rectifier increases with the frequency of the current delivered by the alternator, thereby prompting the use of a small number of poles.
Furthermore, the advantage of operating the voltage-booster chopper with a very high switching frequency is that it reduces the level of the current harmonics and consequently the heating up of the alternator.
There is a need to provide a device making it possible to generate a relatively high power with satisfactory efficiency, the power being typically between 1 and 30 kW, reliable and of low weight, capable of being stowed on board a vehicle or a container for example.
The subject of the present invention is therefore a novel device for producing electricity intended in particular to be stowed on board a vehicle or a container, comprising a three-phase alternator able to be driven in rotation at a variable speed by a thermal engine, a rectifier for generating a rectified current from the current delivered by the alternator and a voltage-booster chopper using the self-inductance of the alternator to step up the voltage, this device being characterized in that the alternator comprises six or eight poles and in that the chopper operates with a switching frequency of preferably between 3 and 15 kHz.
Preferably, use is made of an alternator having eight poles and a switching frequency of less than 10 kHz, preferably less than 8 kHz and preferably still of the order of 7.5 kHz.
The applicant company has found that it was possible to operate the device according to the invention at such a switching frequency with satisfactory overall efficiency, the device additionally being reliable and relatively easy to construct.
Whereas, when increasing the number of poles of the alternator, one might expect a degradation in the performance of the device, on account of the increase in the losses within the magnetic circuit, it transpires that surprisingly the device according to the invention exhibits a cost, weight and efficiency which are fully compatible with its use on a vehicle or a container.
One attempt at explanation is that the increase in the number of poles has made it possible to considerably reduce the value of the inductance of the alternator and to afford a saving in the voltage drop through overlap, hence a reduction in the current and in the joule losses of the alternator, thereby compensating for the increase in the magnetic losses within the alternator due to the higher number of poles than that of conventional on-board alternators and the increase in losses through switching in the chopper.
Preferably, the % self-inductance per phase of the alternator, defined by the formula Lxcfx89IN/V0, where L is the self-inductance of the alternator, xcfx89 the angular frequency, IN the nominal current at a given speed, for example 1500 rpm, and V0 the no-load voltage of the alternator at the said speed, is between 15 and 40%, preferably between 20 and 30%.
In a preferred embodiment corresponding to a power of the order of 10 kW, the voltage being 294 volts between phases at 3000 rpm, the inductance per phase of the alternator is less than or equal to 2 mH, preferably less than or equal to 1.5 mH, this corresponding to a % inductance of less than 30%.
In an embodiment corresponding to a power of the order of 10 kW, the voltage being 294 volts between phases at 3000 rpm, the inductance per phase of the alternator is greater than or equal to 1 mH, this corresponding to a % inductance of greater than 20%, and the switching frequency is of the order of 5 kHz.
The applicant company has found that these inductance and frequency values made it possible, in the case of an eight-pole alternator to maintain the losses due to the current harmonics at a particularly low level.
Preferably, the alternator is of the permanent-magnet type.
Also preferably, the chopper is downstream of the rectifier, thereby making it possible to use just one transistor, for example an IGBT transistor.
This single transistor can be easily cooled by circulating a liquid, whereas in the known devices whose choppers comprise several transistors, this cooling is made more difficult on account of the number of transistors, which may not be mounted on a common cooler without posing insulation problems.
Preferably, the switching frequency is constant, thereby simplifying the control electronics.
In a preferred embodiment, the chopper supplies at least one inverter controlled by a control circuit receiving a cue representative of the speed of rotation of the rotor of the alternator.
Thus, the control circuit can take into account the power available for the apparatuses linked to the inverter, and control their operation accordingly.
The subject of the invention is also an on-board electric generator of power lying between 1 and 30 kW, characterized in that it comprises in combination a three-phase alternator with eight poles of the permanent-magnet type, of % self-inductance per phase of between 20 and 40%, preferably between 20 and 30%, able to be driven in rotation at variable speed by a thermal engine, a rectifier for generating a rectified current from the current delivered by the alternator and a voltage-booster chopper downstream of the rectifier, this chopper using the self-inductance of the alternator to step up the voltage and using a single IGBT transistor operating at a switching frequency of preferably less than 8 kHZ, preferably still of the order of 7.5 kHz, to short-circuit the alternator.